In controlling a machine or the like driven by a servomotor, linear control, such as PI (proportional-plus-integral) control, is employed assuming the machine, as an object of control, to be a non-spring system such as a rigid body.
However, since a robot is generally constructed in the form of a cantilever, it undergoes a large variation of inertia depending on the attitude of the robot, accompanying the change in the response of a servo system. Moreover, a robot has a low rigidity and resulting low resonance frequency due to the effects of the spring element such as the speed reducer and others, and so, in positioning the distal end (tool center point) of a robot, the end of the robot tends to sway even after positioning by servomotor. Conventionally, therefore, variations in the spring system and inertia due to the low rigidity of the robot have been ignored, and instead, the servo gain is reduced for the control, or a timer is activated after the positioning so that operation is performed after the vibration of the distal end of the robot is stopped following the passage of a predetermined period of time.
Thus, the present inventors have proposed inventions in which sliding mode control is applied to a control object driven by a servomotor, in order to prevent a moving part of a machine from vibrating (See Publication JP-A-3-92911 and JP-A-3-118618).
It is known that a combination of the sliding mode control and adaptive control is highly useful, since it provides very high robust properties (i.e., high resistance to parameter variations and high disturbance restraining capability). If this technique is applied to a machine of flexible structure such as a robot, however, estimated parameters (inertia, coefficient of dynamic friction, gravity term, etc. of the control object) will not converge properly, thereby causing the vibration of machine's moving parts.